"What we're pushing for is to get him to come and do that, and it may require that there be increased pressure on him of various kinds in order to do that. We've made it very clear to people that we are looking at increased steps that can help bring about that pressure," he said.

Kerry said that along with the other countries -- which he did not name -- the U.S. was looking to find ways to reignite the diplomatic process to end the conflict in Syria, which has entered its fifth year.

"To get the Assad regime to negotiate, we're going to have to make it clear to him that there is a determination by everybody to seek that political outcome and change his calculation about negotiating,"Kerry said.

"That's under way right now. And I am convinced that, with the efforts of our allies and others, there will be increased pressure on Assad.”